Hydrophobicity is the physical property of a molecule, known as a hydrophobe, that is repelled from a mass of water. The hydrophobic interaction is mostly an entropic effect originating from the disruption of highly dynamic hydrogen bonds between molecules of liquid water by the non-polar solute. By aggregating together, non-polar molecules reduce the surface area exposed to water and minimize their disruptive effect. Thus, the two immiscible phases (hydrophilic vs. hydrophobic) will change so that their corresponding interfacial area will be minimal. This effect can be visualized in the phenomenon called phase separation in a mixture of water with a lipid solution. It can also be visualized in the nature by observing a water droplet on the hydrophobic surface of grass or a leave. A water droplet on a hydrophobic surface is illustrated in FIG. 1, left image. The contact angle θ or Water Contact Angle (WCA) can be studied by analyzing the forces acting on a fluid droplet resting on a solid surface surrounded by a gas.
Hydrophobicity depends not only on the composition of the surface contacting water but also on its physical topography. Wenzel has determined that when the liquid is in intimate contact with a micro structured or rough surface, θ will change to θW*=r cos θ, where r is the ratio of the actual area to the projected area (see FIG. 1, central image). Wenzel's equation shows that micro structuring a surface amplifies the natural hydrophobic tendency of the surface. According to the Wenzel model, the liquid droplet retains contact at all points with the rough hydrophobic solid surface increasing the interfacial energy and the Water Sliding Angle (WSA). The droplets “stick” therefore to the surface. To reduce the surface contact with the hydrophobic film, the droplet decreases its projected base area, increasing the WCA and the hydrophobic character of the film.
Cassie and Baxter found that if the liquid rests on the tops of microstructures (see FIG. 1, right image), θ will change to θCB*=φ(cos θ+1)−1, where φ is the area fraction of the solid that touches the liquid. Liquid in the Cassie-Baxter state is more mobile than in the Wenzel state. The droplet remains on top of the film protrusions, leading to a “slippy” (i.e. slippery) hydrophobicity characterized by low WSA and WCA hysteresis.
The WSA is a dynamic measure of hydrophobicity and is measured by depositing a droplet on a surface and tilting the surface until the droplet begins to slide. In general, liquids in the Cassie-Baxter state exhibit lower slide angles and contact angle hysteresis than those in the Wenzel state.
Contact angle is a measure of static hydrophobicity, and contact angle hysteresis and slide angle are dynamic measures.
The contact angle formed between a liquid and solid phase will exhibit a range of contact angles that are possible, corresponding to a hysteresis. There are two common methods for measuring this range of contact angles. The first method is referred to as the tilting base method. Once a drop is dispensed on the surface with the surface level, the surface is then tilted from 0° to 90°. As the drop is tilted, the downhill side will be in a state of imminent wetting while the uphill side will be in a state of imminent dewetting. As the tilt increases the downhill contact angle will increase and represents the advancing contact angle while the uphill side will decrease; this is the receding contact angle. The values for these angles just prior to the drop releasing will typically represent the advancing and receding contact angles. The difference between these two angles is the contact angle hysteresis. The second method is often referred to as the add/remove volume method. When the maximum liquid volume is removed from the drop without the interfacial area decreasing the receding contact angle is thus measured. When volume is added to the maximum before the interfacial area increases, this is the advancing contact angle.
The hydrophobic state according to the Wenzel model is therefore more “sticky” than the hydrophobic state according to the Cassie-Baxter model which is more “slippy”.
It is commonly acknowledged that a hydrophobic surface is a surface that shows a WCA that is greater than 90° and a superhydrophobic surface is a surface with a WCA that is greater than 150°.
Superhydrophobic surfaces, such as the leaves of the lotus plant, are those that are extremely difficult to wet. Lotus leaves are covered of microscopic cells (3-13 μm) coated by nanoscopic wax crystals (100 nm). The WCA on a Lotus leaf can exceed 150° and the WSA can be less than 10°. This is referred to as the Lotus effect and corresponds to the Cassie-Baxter state.
The micropapillae (16 μm diameter and 7 μm height) and nanofolds (730 nm) present at the surface of the rose petals lead to a superhydrophobic surface with high adhesive force to water, i.e. high WSA, corresponding to the Wenzel state. Rose petals have the ability to grip water droplets in place. The droplets, which remain spherical in shape on the petal surface, do not roll off even if the petal is turned upside down.
Both the lotus leave and the rose petal show a double roughness or hierarchical structure as schematically illustrated in FIG. 2.
Several criteria have been developed for predicting whether the Wenzel or the Cassie-Baxter state will exist. Among those criteria one considers the height of the micro structure and another one focuses on the air-trapping capability under liquid droplets on rough surfaces.
The scientific publication “One-step process to deposit a soft super-hydrophobic film by filamentary dielectric barrier discharge-assisted CVD using HMCTSO as a precursor”, (M. C. Kim, C.-P. Klages, Surface and Coatings Technology 204 (2009) 428-432.), describes the deposition of superhydrophobic films forming a double rough structure by means of a filamentary dielectric barrier discharge operating from 38,000 to 40,000 Hz fed with a cyclic organosilicon precursor, hexamethylcyclotrisiloxane (HMCTSO). The films grown on both silicon wafers (100) and stainless steel are superhydrophobic with WCA of 162° and 158° respectively. The morphology of the films described in this document compared well with the one observed at the surface of the Lotus leaf. This double rough structure is giving rise to the “slippy” superhydrophobic property, also called Lotus-effect. However, this process is relatively slow and the resulting coating can present some structural weaknesses. Additionally, this process does not allow obtaining surfaces with “sticky” superhydrophobic properties.
The patent application published US 2011/0171426 A1 discloses a method for manufacturing a hydrophobic surface. The method consists essentially in applying by atmospheric pressure plasma deposition on a substrate a first coating forming a hard and rough surface and after a second coating on the rough surface. The first coating has a roughness ranging from 9 nm to 1 μm. Like in the preceding teaching, this multi-steps process is relatively slow and does not lead to superhydrophobic surfaces.
The patent application published US 2011/0287203 A1 discloses a method for manufacturing a superhydrophobic and self-cleaning surface. The method involves essentially the imprinting of exposed surfaces with suitable fine-grained and/or amorphous metallic embossing dies to transfer a dual surface structure, including ultra-fine features less than or equal to 100 nm embedded in and overlaying a surface topography with macro-surface structure greater than or equal to 1 micron.